Process for producing electrical resistance heaters



H. H. HIRSCH Nov. 1, 1966 PROCESS FOR PRODUCING ELECTRICAL RESISTANCE HEATERS Filed Dec. 27, 1963 Fig. 2.

/m/em0r Ham/d H. Hirsch by H/s Afro/nay- United States Patent 3,281,924 PROCESS FOR PRODUCING ELECTRICAL RESISTANCE HEATERS Harold H. Hirsch, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed Dec. 27, 1963, Ser. No. 333,986 Claims. (Cl. 29-15562) This invention relates to electric resistance heaters and more particularly to an improved process for prdoucing such heaters and to the heaters produced.

Optimum construction of electric heaters, particularly those operating at comparatively low temperatures, usually requires that uniform radiation of heat occur from the device. Intimate contact between the resist ance heat-producing member and the heat-radiating surfaces is essential to practical, eflicient operation. Additionally, the electric resistance wire or heating element should be protected from atmospheric and chemical attack.

It is a principal object of this invention to provide an improved process for producing electric resistance heating elements.

A further object of this invention is to provide a process for producing long lengths of electric resistance heating elements in which metal powder is compacted into intimate thermal contact around the whole circumference and along the length of the insulated electric resistance wire and at the same time, to form the heat-radiating surfaces.

An additional object of this invention is to provide an improved electric resistance heating element.

Other objects and advantages of this invention will be in part obvious and in part explained by reference to the accompanying specification and drawings.

FIG. 1 is a schematic side elevation of one form of apparatus suitable to effect the process of this invention; and

FIG. 2 is a portion of a heating element produced by the present process illustrating the manner in which the resistance wire is disposed within the compacted particulate metal.

Broadly, the present process comprises providing an electrically insulated electric resistance wire which is in a coil-like or helical form having a number of turns. This resistance wire and each of the turns is surrounded with metal powder and the combination then compacted, as by hot pressing or by hot rolling, into an integral body. The procedure causes the coiled resistance wire to be lengthened by stretching the spacing between turns and the coils flattened out to the point that the wire assumes essentially a sinuous form. If desired, the helically-shaped resistance wire can be flattened slightly prior to being surrounded by the particulate metal and hot pressed to final dimensions.

The present process is amendable to several fabricating techniques. For example, one can use either single or where the element to be produced is large, successive stage pressing, rolling for long, continuous lengths, or even forging. These techniques can be used to produce such items as electric Skillets, sauce pans, sole plates for flatirons, surface plates for stoves and hot plates, ovens and furnace panels, grills and clothes driers, water heaters, space heaters for living areas, etc.

0f the fabricating techniques mentioned earlier, it is felt that rolling is the one with which the present concept is most uniquely suited, since this process enables long lengths of heating element to be produced. When fabricating by rolling, the resistance wire must be in a critical form to enable the powder rolling process to be used without destroying the integrity of the electric resistance wire and its insulation.

Referring to FIG. 1 of the drawings, heating elements are produced by continuously introducing a precoiled electrically insulated resistance heating wire 10 through the feed tube 12 into a quantity of metal powder 11. The particular resistance wire used is not critical since materials now commonly used in industry, such as Nichrome, are suitable. The wire is coiled by wrapping it around a mandrel of preselected diameter, with proper spacing between turns, then is removed from the mandrel and either left as coiled or partially flattened and fed into the compaction area through tube 12. As the powder is compressed in the region beyond the end of tube 12, the coil begins to flatten and extends or undergoes a small amount of increase in spacing between turns. This process continues until the powder nearly reaches full density. Further compression, which occurs up to the point of minimum gap between the rolls, causes solid metal flow to occur and the metal strip actually lengthens. This produces longitudinal tensile stresses in the metal and in the wire, particularly where it is aligned in the direction of rolling. The wire or insulation, in the absence of the coil configuration, would undergo longitudinal tensile failure rendering the strip useless for the intended purpose. However, if the coiled wire is made properly, the turns will still be in the form of a stretchable helix during this critical stage and the compression and tensile elongation will be absorbed by the flattening and extension of the coil. What issues from the rolls, then, is a fully compacted metal strip in which is intimately embedded a continuous, insulated resistance wire, sinuous in shape across the width of the strip and essentially lying within a plane perpendicular to the face of the strip. The compacting process automatically centers the wire in the thickness direction. Feed tube 12 serves a dual function of confining and restricting the extension and flattening of the coils to the zone where compaction of the powder takes place, and it also properly centers the wire across the width of the compacted strip. Clearly, multiple tubes could be used where more than one wire coil is to be embedded.

The outside diameter of the coil, whether it is a full circle or partially flattened, must be small enough to allow it to be covered with powder in the space provided at the end of the tube 12. Using as large a diameter coil as possible, within these limitations, has the advantage of producing large amplitudes in the embedded wire, thereby providing more resistance per unit length of strip and provides further improvement in over-all heat transfer. This coil diameter can be as small as 0.5 times up to as much as 2 to 3 times the final strip thickness, and is dictated by the apparent density of the powder and the final strip thickness. The latter is actually determined more by the total thickness of the resistance wire plus insulation and must be great enough to completely cover the wire. With the process described herein, the final strip thickness should be at least 40 mils greater than the starting diameter of the insulated wire to completely mask the presence of the wire.

Any insulation can be used on wire 10 which will withstand the temperature at which the powders must be hot compacted and the subsequent operating temperature. Thus, with aluminum, the insulation should be capable of withstanding temperatures up to 600 C. for short periods of time and somewhat lower temperatures for many thousands of hours. The insulation must also be capable of withstanding the mechanical bending, compression and stretching that occur during the various stages of processing. Coatings such as braided, wrapped or knitted fiber glass or felted asbestos are suitable.

To produce final strip, metal powder 11 is fed into the hopper 15 and wire it is simultaneously fed into the powder through guide tube 12. The tube I12 ends just above compaction rolls 13 so that the turns of the wire are surrounded by metal powder for only a short distance above the rolls. This arrangement is important to prevent compaction from occurring too far up within hopper 15 which could result in either rupture of wire 10 or its insulation. Rolls 13 compact the powder and effect flattening and lengthening of the wire 10 in the manner shown in FIG. 1. Referring to this FIGURE, it can be seen that the wire 10, which started out as a helix having a number of turns, is changed from this helical form as it is flattened out and stretched during its passage between rolls 13. After coming from the rolls, the form becomes generally sinuous as shown in FIG. 2, although some small vertical displacement may remain.

It is important to the successful operation of the process that adequate open space be left between successive turns of the helically-shaped Wire 10. Specifically, it has been found that generally an open space of not less than inch and not more than of an inch must be provided if successful production is to be achieved. Openings outside of this range result either in rupturing of the wire or of the insulation, either situation, of course, rendering the resulting product useless. Further, since the amount of space between successive turns of the helix is important, it is also important that the particle size and shape of the metal powder be such that it flows freely and possesses a constant consistency so as to ensure that the powder will completely surround each of the turns of the coil and that a uniform strip or sheet of high density is obtained. A smooth, rounded particle shape is recommended, falling within a 6 to 100 mesh size range, although finer powder could be used providing it is free flowing. Any type of metal in powder form can be used depending upon the ultimate use contemplated for the resulting heater. Aluminum, however, is particularly attractive for lower temperature heaters since it can be readily prepared into the proper type of powder, it can be heated in air without undergoing excessive oxidation, and it can be compressed into high density, high strength strip under manageable temperature and pressure conditions. In addition, aluminum possesses a high thermal conductivity and has good chemical stability in normally encountered atmospheres. Pure aluminum can be used or the commercially available aluminum alloys are equally suitable.

Although the present invention has been described in connection with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A process for manufacturing electrical resistance heating elements, said process comprising providing an electrically insulated electric resistance wire shaped in the form of a helix having a plurality of turns, surround- .4 ing the helix and each of the turns thereof with thermally conductive metal powder, and compacting the metal powder and simultaneously flattening the turns of the helically-shaped resistance wire into flat sinuous form, thereby densifying the metal powder and elongating and flattening the helix.

2. A process for manufacturing electrical resistance heating elements, said process comprising providing an electrically insulated electric resistance wire shaped in the form of a helix having from about A to inch separation between the turns thereof, surrounding the helix and each of the turns thereof with thermally conductive metal powder, and compacting the metal powder and simultaneously flattening the turns of the helicallyshaped resistance wire into flat sinuous form, thereby densifying the metal powder and elongating and flattening the helix.

3. A process for manufacturing electrical resistance heating elements, said process comprising providing an electrically insulated electric resistance wire shaped in the form of a helix having a plurality of turns, surrounding the helix and each of the turns thereof with thermally conductive metal powder having a particle size within the range of from 6 to mesh, and compacting the metal powder and simultaneously flattening the turns of the helically-shaped resistance wire into flat sinuous form, thereby densifying the metal powder and elongating and flattening the helix.

4. A process as defined in claim 1 wherein the diameter of the helix initially ranges from about 0.5 to 3.0 times the final thickness of the heating element.

5. A process for manufacturing electrical resistance heating elements, said process comprising providing an electrically insulated electric resistance wire shaped in the form of a helix having a plurality of turns, feeding the wire into compaction rolls and surrounding it with metal powder for only a short distance above the rolls, said feeding being through a tube which ends just above the compaction rolls, and compacting the metal powder and simultaneously flattening the turns of the helically-shaped resistance wire into flat sinuous form, thereby densifying the metal powder and flattening the helix.

References Cited by the Examiner UNITED STATES PATENTS 1,359,400 11/1920 Lightfoot 29l55.5 1,933,128 10/1933 Wiegand 338269 2,643,317 2/1950 Tuttle 29155.63 2,735,162 2/1956 Huck 29155.63 2,789,926 4/1957 Finholt et al. 15651 2,933,805 4/1960 McOrlly 338238 2,959,756 11/1960 Lennox 338243 3,007,235 11/1961 Yohe 338238 3,017,688 1/1962 Cummings et al. 29155.66 3,065,436 11/1962 Kayko et al. 338243 3,123,898 3/1964 Bremer 29-l55.63

JOHN F. CAMPBELL, Primary Examiner.

I. M. ROMANCHIK, JR., Assistant Examiner. 

1. A PROCESS FOR MANUFACTURING ELECTRICAL RESISTANCE HEATING ELEMENTS, SAID PROCESS COMPRISING PROVIDING AN ELECTRICALLY INSULATED ELECTRIC RESISTANCE WIRE SHAPED IN THE FORM OF A HELIX HAVING A PLURALITY OF TURNS, SURROUNDING THE HELIX AND EACH OF THE TURNS THEREOF WITH THERMALLY CONDUCTIVE METAL POWDER, AND COMPACTING THE METAL POWDER AND SIMULTANEOUSLY FLATTENING THE TURNS OF THE HELICALLY-SHAPED RESISTANCE WIRE INTO FLAT SINUOUS FORM, THEREBY DENSIFYING THE METAL POWDER AND ELONGATING AND FLATTENING THE HELIX. 